1. Field of the Invention
The present invention relates to a camera capable of driving a photographic lens by performing automatic focus detection in multiple focus detection regions and, more particularly, to providing a smooth, accurate drive of the photographic lens with automatic focus detection in multiple focus detection regions.
2. Description of the Related Art
Conventional cameras can automatically adjust the focal point of a photographic lens to determine an appropriate focus position of the photographic lens. Typically, automatic focus point adjustment is performed for a single focal point detection area located in the center of the viewfinder display. Single automatic focus (AF) servos and continuous AF servos have been used for such focal point adjustment operation.
With a single AF servo, the subject is brought into focus by detecting the focus position of the photographic lens in the focal point detection area and a "focus lock" is applied to prevent subsequent movements of the photographic lens. If a subject is in a location other than the center of the viewfinder display, the subject can be maintained in a state of focus by arranging the subject to be in the center of the viewfinder display, focusing the photographic lens, setting the focus lock, and then changing the composition in the viewfinder display with the focus lock set so that the subject is moved to a different portion of the viewfinder display. Thus, the focus is set to the distance of the subject, but, as desired by the photographer, the subject is no longer positioned in the center of the viewfinder display.
By contrast, a continuous AF servo operates to continuously execute focus point detection so that the photographic lens is driven as necessary to focus the subject. The camera detects a change in the focus position of the photographic lens due to the movement of the subject or a change in the composition in the viewfinder screen. Based on the detected change, the photographic lens is continuously driven to always maintain focus.
Moreover, conventional cameras are provided with automatic focal point adjustment for multiple focal point detection areas. Japanese Patent Publication HEI-1-279215 discloses such a camera in which respective focal point detection areas are selected by a photographer operating an area selection switch.
In conventional cameras provided with automatic focal point adjustment for multiple focal point detection areas, a smooth drive of the photographic lens is not provided since a new distance measurement (that is, a measurement of the distance from the camera to the subject) is performed each time the focal point detection area is switched to a new focal point detection area. No consideration is made of any distance measurement performed prior to switching to the new focal point detection area. Moreover, responsiveness is reduced if data must be obtained and stored for multiple distance measurements in order to obtain a new focus point after switching focal point detection areas.
For example, there is a high probability that unstable operations will arise and unnecessary drive of the photographic lens will be performed during continuous AF servo operations if the focus is switched from a first focal point detection area to a second focal point detection area and the distance to the subject in the first focal point detection area is significantly different from the distance to the subject in the second focal point detection area. Moreover, conventional cameras typically switch from one focal point detection area to a different focal point detection area by sequentially switching the focal point detection areas in a specific order. Thus, unsatisfactory operation can occur while the photographer switches through the various focal point detection areas in an attempt to locate a desired subject or a desired focal point detection area.